This invention relates to the field of cathode ray tube (CRT) testing and reconditioning. More particularly, this invention relates to a system for testing both monochrome and color CRTs for acceptable electron emission from the cathode element and for non-invasively reconditioning a weak cathode element, when required.
The cathode ray tube, because of its inherent nature, is one element of a computer system, a television set, or test equipment that is virtually guaranteed to "wear out" with use. The emissive material coating the cathode or cathodes of most CRTs is subject to a "crusting" phenomenon, which is believed due to several factors, such as depletion of the emissive cathode coating or oxidation of the emissive cathode coating from gasses remaining in the tube after manufacture. Regardless of the cause, the net result is a reduction in cathode emissions, which impairs the ability of the CRT to provide sharp images with acceptable contrast and resolution.
Several methods are known which have been devised to extend the useful life of the CRT by reversing the "crusting" phenomenon. One popular approach is known as the Beltron process which is basically described in U.S. Pat. No. 3,641,391 and which provides a workable methodology for the testing, cleaning and restoring of a CRT (when necessary). Prior to the advent of the invention herein, the Beltron process was implemented in a portable carrying case having a hinged lid for protecting the test equipment panel and associated hardware components. In this version, the test equipment includes several analog meters and indicator lamps, manually controllable switches and potentiometers, and three separate connector sockets for connecting a CRT to the test equipment during three different modes of operation: viz., Test, Clean and Restore. The test equipment includes suitable circuitry for generating the necessary AC and DC voltages required to perform the various modes of operation in the following manner.
Initially, the operator connects a CRT to the test socket using a cable connector and adapter socket appropriate for the particular CRT to be tested, and the unit is turned on to apply AC power to the system circuitry. Next, the operator manually adjusts the potentiometer which controls the value of the filament DC voltage, observing the value of the voltage on one of the analog meters. At the same time, the operator observes one or more of the analog current meters to observe the rise time of each cathode/grid circuit. During this portion of the test process, the application of the proper filament voltage to the filament of the CRT causes heating of the cathode element(s) within the tube. As the cathode is heated, the current flowing between the cathode and the grid is measured by the meter connected to the particular cathode/grid circuit. For a tube functioning adequately, the rise time from zero to a preselected threshold value (about 85 milliamps) should not exceed about 15 seconds. For a color CRT with three sets of cathode/grid elements, all three pairs of elements should exhibit approximately the same rise time. This can be observed by the operator by looking at the three separate ammeters individually dedicated to the three gun elements. After noting the rise time, the operator then interrupts the filament DC voltage using an interrupt switch and observes the emission time by looking at the same ammeters and noting the length of time that each pair of cathode/grid elements remains at the threshold current level before beginning to decay. For a good tube, the steady state maximum threshold current should remain for approximately 8 to 10 seconds, and then smoothly drop off to zero milliamp. After this part of the test process, the operator then releases the interrupt switch and again observes the current rise time. During the test process, a filament cathode short is signified by the illumination of a dedicated SHORT lamp. For a CRT which exhibits acceptable rise time and emission time characteristics (as observed by the operator) and which does not exhibit a filament/cathode short, no further procedures are necessary under the Beltron process.
For a CRT which does not test up to the desired standard, the Clean procedure is next performed. Initially, the operator removes the connector from the test socket and installs the connector in the dedicated Clean socket. Next, the operator manually increases the filament voltage by approximately 60% to a value of approximately 10 or 20 volts (depending on whether the CRT has a 6.3 volt or a 12.6 volt filament) and observes the cleaning lamps. During the cleaning process, the test equipment applies an AC voltage across the cathode/grid elements. The elevated filament voltage heats the cathode beyond the normal range and boils off some of the non-emissive material from the cathode. The AC voltage assists in this process and also helps to remove any contaminant materials or particles which might be lodged between the cathode and grid elements. During this process, the operator observes the cleaning lamps. After approximately 30 seconds all lamps should glow brightly. Next, the operator reduces the filament voltage to the normal value and, after a 30 seconds cool down period, removes the connector from the clean socket, reinstalls the connector to the test socket and repeats the test procedure. During the clean procedure, if any cleaning lamps do not glow brightly at the elevated filament voltage, or are extinguished when the filament voltage is reduced to the nominal value, then the Restore procedure is commenced.
To begin the Restore procedure, the connector is installed in the Restore socket. For a color CRT, a three position gun selector switch is manually set to one of the indicated gun positions which has been determined to require the Restore procedure during the Test and Clean procedures. After the setting of the gun selector switch, the operator manually increases the filament voltage by approximately 60%. During the Restore procedure, the elevated filament voltage heats the cathode beyond the normal operating temperature to burn off non-emissive contaminants (as in the Clean procedure). At the same time, a relatively high DC voltage is applied between the cathode and grid elements, which helps attract electron emissive material to the cathode surface. During the Restore procedure, the operator observes the restoring current meter associated with the selected gun element. The meter should show a smooth increase of current to at least about 80 milliamps. At the same time, the operator observes a restoring indicator lamp which should glow in unison with the meter. When the Restore current meter indicates that the cathode grid current has reached the 80 milliamp value, the Restore circuit is interrupted by the operator by depressing an interrupt switch. After a few seconds, the operator then releases the interrupt switch to reapply the Restore current. A successfully restored gun should show a smooth increase on the Restore current meter to at least 80 milliamps with the restoring indicator lamp glowing steadily in unison with the meter. After the Restore procedure has been completed for one gun element, the gun selector switch is manually set to the position of the next gun element requiring the Restore procedure. The procedure is sequentially repeated for those gun elements requiring the Restore procedure. After the Restore procedure has been completed for the last gun element, the CRT is permitted to cool for at least 30 seconds. Thereafter, the Test procedure is repeated for the CRT.
After a successful Restore procedure, the CRT is finally balanced by returning to the Clean procedure. As the operator manually increases filament voltage in a gradual fashion up to the increased 60% level, the cleaning lamps are observed. All three cleaning lamps (one for a monochrome CRT) should glow brightly. After approximately 10 seconds of the elevated filament voltage, the operator reduces the filament voltage to the normal value. The cleaning lamps should continue to glow for several seconds. Thereafter, the filament voltage is reduced to zero and the tube is allowed to cool for at least 30 seconds. The operator then returns to the Test procedure to do a final test on the now-restored CRT.
While the above Beltron process has been successfully used for many years for testing, cleaning and restoring those CRTs capable of being restored, the system used to implement the process has several drawbacks. Firstly, the necessity for manually disconnecting the CRT connector from one socket and connecting this to a different socket is time consuming, which slows down the entire process. In addition, the need to visually observe several different meters while manually adjusting the filament voltage introduces an element of subjectivity into all three procedures (i.e., Test, Clean and Restore) which can lead to varying results and renders the system less useful to those of only minimal technical skill.